The present invention relates to an hydraulic elevating mechanism for lifting and lowering an engineering structure sluingly supported around a rotational axis on a base unit and comprising two or more hydraulic lift cylinders pivoted to the base unit at one end and to the engineering structure to be slued at the other.
Elevating mechanisms of this sort are preferably employed in agricultural machines such as harvester threshers, mowers, potato and beet lifters and the like.
In the following, the invention is described in more detail based on a harvester thresher having a sluable cutter table as representing all such machines.
In the operation of a harvester thresher, the cutter table of this machine, in the majority of cases, has to be so guided over the ground that it will not dig into the soil but on the other hand will support its residual weight on the ground in order to follow the unevennesses of the ground and cut the wheat at a constant height.
In practice, unfortunately, difficulties are experienced in guiding the cutter table which is sluingly supported around a generally horizontal axis on the harvester thresher, over the ground in the way described in the foregoing. The major part of its weight is supported by one or a plurality of hydraulic cylinders which are pivotably supported at the cutter table on one hand and on the undercarriage on the other, rendering height adjustment of the cutter table possible. In order to be able to guide the cutter table with its residual weight so over the ground that it follows ground unevennesses without digging into it, spring elements of most different kind are employed. The most common practice is to provide a spring arrangement directly in the pressure medium which actuates the hydraulic cylinder. The spring arrangement consists of a gas volume of a predetermined amount introduced into the hydraulic system and is compressed by the hydraulic liquid under pressure. If the pressure in the hydraulic system decreases, for instance if he cutter table travels over a ground wave, the gas volume under pressure causes that, without actuating the lift valve, the major part of the cutter table weight is supported via the hydraulic cylinder(s) on the undercarriage and only a residual weight is supported on the ground.
The disadvantage of such an hydropneumatic pressure reservoir arranged in the hydraulic system is that this spring arrangement possesses a very steep spring characteristic, i.e. the hydraulic pressure significantly decreases already at a minute uplift of the cutter table so that the residual weight resting on the ground is substantially increased and the cutter table tends to dig into the ground.
In order to maintain the spring characteristic more flat, a pressure reservoir having a very large volume would have to be provided. This however has the disadvantage that the cutter table does not only spring upwards but also into the opposite direction, that is, locks downwardly if an additional load, for instance incoming straw, acts onto the cutter table. This is very much undesirable. Efforts are therefore directed towards permitting the cutter table to spring up with a very flat spring characteristic but to arrest it in the downward direction, if possible, in a non-lockable yet adjustable point.
In practice, it is for instance desirable to adjust the cutter table to a height of about 20 cm above ground whereby it does not lock in downwardly but on the other hand can spring upwardly without any great energy consumption.
In applicant's prior patent application, serial no. P 25 02 666.8, an hydraulic spring arrangement has been described which eliminates the disadvantages described in that a control cylinder is inserted between lift cylinders and pressure reservoir. This renders it possible to adjust the cushioning pressure and to lock he downward spring arrest of the cutter table, while spring-off upwardly is fully maintained.
A further method to cushion the cutter table weight consists in connecting one, or a plurality of mechanical springs to, or on, the lift cylinders which will then cushion the cutter table weight.
Although various different structures of this kind have been known, a completely satisfactory solution has hitherto not been found. The simplest way of mechanical cushioning consists in the provision of a helical spring between cutter table and lift cylinder. Such helical spring however has to be exactly adapted to the weight of the cutter table. It has to block as soon as the cutter table is lifted above the ground, i.e. to press together so far that one spring coil touches the next spring coil and the cutter table thus cannot lock downwardly. Upwardly, however, the spring force should be fully available. Since for the efficient work on the field, the light spring-off mentioned above is a pre-requisite, the spring characteristic has to be very flat. This leads to very large helical springs which have to be individually designed for each harvester thresher with respect to the different weights of the various cutter table types, which leads to a plurality of spring embodiments. This way of cushioning has the additional disadvantage that when travelling on a road, the spring effect cannot be eliminated, nor limited or reduced, which leads to major oscillations of the cutter table when travelling on uneven field paths.
Since the spring can be compressed only by the weight of the cutter table, it is further on not possible to make use of this kind of spring action in the case of harvester threshers provided with removable cutter tables. If the weight of the cutter table is lacking, the springs extend that far that the cutter table cannot be coupled in again.
In order to avoid these disadvantages, the springs are so arranged that the weight of the cutter table rests on the springs only if the cutter table is lowered. With one end, the spring rests against one outer support of the cylinder tube of the lift cylinder and with the other end on an outer support of the piston rod, where at least one of the outer supports is axially variable by means of a thread or a bracing clamp for the respective adjustment of the spring.
The disadvantage of this structure on one hand is that also in this case very large springs having a correspondingly flat spring characteristic have to be used in order to have available a desired large spring path of the cutter table and on the other hand that the spring adjustment for coupling the cutter table on and off has to be adjusted by hand to allow the conveyor channel of the harvester thresher to be lowered far enough for this coupling action while the weight of the cutter table is not available. This however requires too much of the time which is particularly scarce during harvest time.
During cutting action, the spring cannot be adjusted corresponding to the requirements; depending on the table height adjusted, the spring force therefore changes too much.
In order to obtain the required tensile path for this kind of cushioning, the piston rod, in its completely drawn-in state has to extend considerably out of the lift cylinder. This requires a substantially longer structure of the cylinder to make corresponding lift heights possible. This elongated structure however impairs the desired short pivot of the cutter tables so that constructional disadvantages have to be accepted.
In a further prior art embodiment of the cutter table cushioning by mechanical springs, the cutter table is not cushioned relative to the lift cylinder, the latter is rather so included into the cushioning that cutter table and lift cylinder are cushioned relative to the undercarriage. While this has the advantage that no breakaway forces have to be overcome in spring operation, the disadvantage is that the spring is constantly biased relative to the cutter table weight.
If the cutter table weight is varied by additional implements such as rope attachments, devices for sun flower harvesting or the like, the tension of the spring, too, has manually to be varied. In the majority of cases, this is done by cocking screws which, however, with a view to the required large spring force, are under an enormous tension, and substantial consumption of energy and time is required to adjust such cocking screws. It is obvious that such adjustment cannot be performed during the operation of the machine.
In one embodiment of applicant's prior patent application No. P 24 18 232.9, a pneumatic spring implement is described where the hydraulic lift cylinder is cushioned relative to the undercarriage by means of a pneumatic diaphragm cylinder. The diameter of the diaphragm cylinder in that case is substantially larger than the diameter of the lift cylinder and can, therefore, contrary to direct cushioning of the hydraulic medium by pneumatic pressure reservoirs, be operated with lower pressure. This low pressure has the advantage that atmospheric air can be used as pressure medium and the pressure in the pressure reservoir can be varied by means of conventional compressors as are commonly used in trucks, gas stations and workshops.
Since however harvester threshers and other working machines do not, as a rule, include compressor plants, the shock-absorber pressure once determined cannot be changed during operation. But even if a compressor were available, a rapidly desired increase of the shock-absorbing pressure takes too long since the pressure in the complete reservoir volume has to be increased.
In applicant's prior patent application No. P 27 28 199, a lift unit has been described, where the relief spring is also provided between undercarriage and cylinder unit. The cylinder unit consists of the proper lift cylinder and a cocking cylinder provided around the lift cylinder, where the lift cylinder, so to say, serves as cock cylinder. The bias of the spring can therefore be adjusted by means of corresponding control elements. The weight of the cutter table is supported via the lift cylinder, the cock cylinder and the spring on the undercarriage. The spring-off of the cutter table is made possible in that the cylinder unit can axially move at a predetermined amount within a bearing block provided on the undercarriage. This axial path however has to be restricted, otherwise the cutter table will oscillate very strongly, for instance when travelling on an uneven field path.
In combination with applicant's prior patent application No. P 25 29 160.5 wherein a circuit has been described wherein the length variation of a spring triggers corresponding switching functions to feed to, or drain, pressure medium from the lift cylinder, the spring path allowing only small spring force variation can be designed very extensive. The disadvantage of this unit however is that a complex guidance of the lift cylinder unit on the undercarriage is necessary which has also correspondingly to be maintained with grease.
The greatest disadvantage however, as mentioned above, is that the cutter table when travelling on the road can oscillate about an amount, and above all includes a lift retardation, which can correspond to the predetermined axial spring path of the lift unit if the cutter table touches the ground. This is of a particular disadvantage if, because of a sudden obstacle, the operator has to lift the cutter table very suddenly, the latter however will react only when the lift unit has performed the predetermined axial path mentioned above.
In applicant's prior patent application No. P 41 26 515.7, an elevating device has been described which is characterized in that a mechanical spring element having one or a plurality of cocking cylinders bridging the normal lift cylinder is so arranged that the bias force of the mechanical spring can be varied by means of the cocking cylinders.
While this structure, in combination with a corresponding electro-hydraulic control, responds to the requirements of a functioning cutter table system, it is relatively complex and hence cost-intensive. A further disadvantage is the voluminous structure of these elevating mechanism which alone accounts for the structural disadvantages.
The present invention now aims at providing a cost and space saving elevating-spring-mechanism for the cutter table which, in addition, fulfills all functional requirements.
This problem is solved by means of an hydraulic elevating mechanism of the kind defined in the beginning which is characterized in that the lift cylinders are controlled by hydraulic control circuits separated from each other and that at least one of these lift cylinders is connected to a pressure reservoir. The structure according to the invention therefore comprises separately provided hydraulic lift and spring cylinders having separate hydraulic control circuits each, whereby the hydraulic medium is cushioned in one or a plurality of lift cylinders by pressure reservoirs and is not, or only insignificantly, cushioned by means of one or a plurality of similarly pivoted and, if possible, constructionally identical hydraulic cylinders. For the spring action in the case of such a combined structure, a very large volume pressure reservoir can be used which therefore has a very flat spring characteristic and renders correspondingly long spring paths of the employed hydraulic cylinders possible while no pressure medium is supplied or drained.
By means of these hydraulically cushioned cylinders, the cutter table is guided in soil contact and hence with the supporting residual weight above the ground and is adjusted relative to the harvester thresher by the lift cylinder(s) not cushioned or only insignificantly cushioned, if mowing without ground contact is to be performed. Hydraulic control is performed in the cushioned hydraulic cylinders by the changing pressure in the hydraulic medium. This pressure, again, can be varied by corresponding control elements, as described further on, in response to the desired ground contact pressure. If mowing is to be performed above ground, that is not in soil contact, the lift cylinder(s) not hydraulically cushioned arel employed for controlling the cutter table. While, in this way, the cutter table can deviate upwardly via the hydraulically cushioned lift cylinders, a downward stroke is locked.
Since when lifting the table the cushioned cylinders support the lift cylinders, it has shown to be sufficient in most of the cutter tables presently employed to install two cushioned cylinders and one lift cylinder. It is of course possible to also employ a plurality of cushioned cylinders and a plurality of non-cushioned lift cylinders. The weight of the respective cutter table is decisive for the number and arrangement of the different cylinders respectively required.
In the following explanation based on a drawing of an embodiment according to the invention, an elevating mechanism comprising two lift cylinders hydraulically cushioned and one lift cylinder not cushioned or only insignificantly cushioned is described.